The current revolution in wireless communications and the need for smaller wireless communications devices has spawned significant efforts directed to the optimization and miniaturization of radio communications electronic devices. Passive components, such as inductors, capacitors and transformers, play a necessary role in the devices' operation and thus efforts have been directed toward reducing the size and improving the performance and fabrication efficiency of such components.
An inductor is an electromagnetic component employed in alternating current and radio frequency applications such as oscillators, amplifiers and signal filters, to provide frequency dependent effects. A discrete conventional inductor comprises a plurality of windings typically enclosing a core constructed of magnetic material. Use of a magnetic core yields a higher inductance value, but is not necessarily required. The inductance is also a function of the number of coil turns (specifically, the inductance is proportional to the square of the number of turns) and the core area. Conventional discrete inductors are formed as a helix (also referred to as a solenoidal shape) or a torroid. The core is typically formed of a ferromagnetic material (e.g., iron, cobalt, nickel) having a plurality of magnetic domains. The application of a magnetic field to the core material when the inductor is energized causes domain alignment and a resulting increase in the material permeability, which in turn increases the inductance.
With the continued expansion of communications services into higher frequency bands, the inductors are required to operate at higher frequencies. But it is known that inductor losses increase as the operational frequency increases due to larger eddy currents and the skin effect. To avoid these losses at relatively low operational frequencies, the inductive effect can be simulated by certain active devices. But the active devices cannot provide acceptable inductive effects at higher frequencies, have a limited dynamic range and can inject additional unwanted noise into the operating circuits.
Forming inductors in semiconductor circuits can be problematic, especially as integrated circuit size shrinks to improve device performance. Compared with current device sizes and line widths, inductors and capacitors are large structures that consume valuable space on the semiconductor surface and are therefore not easily integrated into semiconductor devices. Ideally, the inductors should be formed on a relatively small surface area of a semiconductor substrate, using methods and procedures that are conventional in the semiconductor processing art. Further, the inductor must be operational at the high frequencies used in today's communications devices and exhibit limited power losses.
Typically, inductors formed on an integrated substrate surface have a spiral shape where the spiral is in a plane parallel to the substrate surface. Many techniques are known for forming the spiral inductor, such as by patterning and etching a conductive material formed on the substrate surface. Multiple interconnected spiral inductors can be formed to provide desired inductive properties and/or simplify the inductor fabrication process. See for example, U.S. Pat. No. 6,429,504 describing a multi-layer spiral inductor and U.S. Pat. No. 5,610,433 describing a plurality of spaced-apart stacked circular conductors interconnected by vias to form a plurality of spiral conductors.
Problems encountered when forming an inductor on the surface of a semiconductor substrate include self-resonance caused by a parasitic capacitance between the (spiral) inductor and the underlying substrate, and the consumption of excess power by the conductor forming the inductor and the inductor's parasitic resistance. Both of these effects can limit the high frequency performance of the inductor.
The Q (quality factor) of an inductor is a ratio of inductive reactance to resistance and is related to the inductor's bandwidth. High Q inductors (i.e., exhibiting a relatively low inductive resistance) present a narrow Q peak as a function of the input signal frequency, with the peak representing the inductor resonant frequency. High Q inductors are especially desirable for use in frequency-dependent circuits operating with narrow bandwidths. Because the Q value is an inverse function of inductor resistance, minimizing the resistance increases the Q.
One technique for minimizing the resistance increases the cross-sectional area of the conductive material forming the inductor. However, increasing the cross-sectional area increases the conductor aspect ratio (i.e., the ratio of the conductor height above a semiconductor substrate plane to the conductor width along the plane). Such high aspect ratio conductors formed on the semiconductor substrate can lead to difficulties in subsequent etching, cleaning, and passivating processes due to steps formed between an upper surface of the relatively thick conductor and an upper surface of the substrate. Such inductors also consume valuable space on the semiconductor substrate. Formation of high aspect ratio inductors can also promote dielectric gaps, which may lead to device failures, between the inductor's closely spaced conductive lines. Although there are known processes for attempting to fill these gaps, such processes are not always successful.